Numerous factors have combined to create a need for increasingly accurate and repeatable stress and vibration testing of structures and devices. Among these are the trend to build things lighter and stronger, the increasing usage of new and untested materials, and an increasing awareness of the need for predictability and safety in the design and manufacture of products. Therefore, the field of vibration testing is rapidly advancing. Vibration testing is performed on actual items of manufacture where size and economy permit. Where this is not feasible, vibration testing may be performed on scale models or mock ups of items thought to have the same relative resistance to vibration as the actual items of interest.
U.S. Pat. Nos. 3,710,082, and 3,848,115, both issued to Sloane et al., are concerned with the process of controlling essentially random signals with the objective of maintaining an overall spectral density of a vibration pattern within acceptable limits. Such an approach recognizes the random nature of many naturally occurring vibration sources. While this conceptual approach is perfectly valid and correct, it has been recognized that a more precisely defined stimulus might lead to a higher degree of repeatability in testing. One approach that has been tried is to use motive stimulus defined by sine waves. Any complex wave can be synthesized using a combination of sine waves. Therefore, a multiexciter system with each exciter being driven by precisely controlled sums of sine waves could, theoretically, produce any desired complex vibration pattern in a structure. In a multiexciter system, stimulus, and response are best described by vectors of dimension N and impedance factors are best described by a matrix of N by N dimensions, with N being the number of stimulus/response points involved.
The objective of a multiexciter swept-sinewave test is to impart a controlled stimulus to a structure at specified points via a series of actuators. A desired stimulus can be represented as a complex vector spectrum. A multiexciter controller, through feedback, continuously excites the structure, measures the response spectral vector at the control points, and modifies the drive signal spectral vector until the response vector agrees with the desired stimulus vector to within some acceptable error tolerance.
In the past, these tests have been performed using purely analog means. In the analog systems, phase relationships between response points were controlled by inducing phase shifts between the drive signal components as a function of the phase difference between the response points. However, the analog approach proved to be largely unsuccessful at frequencies near structural resonance frequencies, since cross coupling effects between the drive signal components and the structure's frequency response characteristics were not accounted for.
More recently, digital approaches have been tried with greater success. The most important reason for the success of digital control systems in these applications is that digital systems can employ a feedback control algorithm that accounts for structural cross coupling effects by using a structural frequency response matrix measured before the test. U.S. Pat. No. 4,782,324, issued to the present inventor, teaches a method and apparatus for converting a digital signal into an analog signal useful for vibration exciter stimulation.
However, even the currently available digital control systems will not provide the desired degree of control when applied to nonlinear and/or time varying systems because the frequency response matrix estimate used by the control system may differ from the actual frequency response matrix existing during the test. For instance, nonlinear stiffness effects will generally cause a shift in the resonant frequency that will cause a large deviation in the phase of the measured frequency response matrix as compared to the response matrix which the control system actually encounters as it is conducting the swept sine test. These potentially large phase discrepancies can cause control system instabilities. Further, imperfections in controller drive and response circuits can lead to undetected inaccuracies in conventional systems. Clearly, there is a need to be able to dynamically compensate for nonlinear and time variant deviations in a structure response matrix during vibration testing, and to detect and correct for other system inaccuracies.
All of the prior art systems for digitally controlling multiexciter swept-sinewave vibration testing within the inventor's knowledge have employed a predetermined and set structural frequency response matrix.
No prior art controller to the inventor's knowledge has successfully compensated for non linear or time variant factors in a system frequency response matrix. All multiexciter swept-sinewave vibration test controllers to date have suffered a high degree of inaccuracy or instability when encountering such variable factors, especially near structural resonance frequencies.